Discharge lamp which incorporates cerium and cesium halides and a high mercury loading

ABSTRACT

Arc-discharge device wherein the discharge-sustaining filling includes as essential elements predetermined amounts of at least one of praseodymium halide, neodymium halide and cerium halide plus cesium halide and sufficient mercury to provide an operating mercury-vapor pressure of from 3 to 15 atmospheres, in addition to the usual starting gas. The rare-earth metal halide provides a very efficient discharge and the cesium halide plus the high mercury loading permits a high efficiency to be obtained with a relatively low minimum envelope temperature. Other dischargesustaining materials desirably are added to the foregoing essential constituents to modify the color of the discharge.

Q United States Patent [1 1 [Ill 3,786,297 Zollweg et al. 1 Jan. 15, 1974 [54] DISCHARGE LAMP WHICH 3,334,261 8/1967 Butler et al. 313/229 INCQRPORATES CERIUM AND CESIUM fl arson HALIDES AND A HIGH MERCURY 3,530,327 9/1970 Zollweg et al. 313/229 LOADING [75] Inventors: Robert J. Zollweg, Monroeville, Primary Examiner-Roy Lake Kenneth K. Blackham, Walter J, Assistant ExaminerDarwin R Hostetter Burnham, both of Pittsburgh,all y Stratum 61 a]. of Pa.

[73] Assignee: Westinghouse Electric Corporation, ABSTRACT Pittsburgh, Pa. Arc-discharge device wherein the discharge-sustaining [22] Flled' 1972 filling includes as essential elements predetermined [21 App]. NO I 243,711 amounts of at least one of praseodymium halide, neodymium halide and cerium halide plus cesium halide Related Apphcatmn Data and sufficient mercury to provide an operating mercu- [63] Continuatiorvin-part of Ser. No. 108,427, Jan. 21, ry vapor pressure of from 3 to 15 atmospheres, in ad 1971 abandoneddition to the usual starting gas. The rare-earth metal halide provides a very efficient discharge and the ce- [52] U.S. Cl 313/229, 3l3/l84, 313/225 Sium halide l the high mercury loading permits a [51] Int. Cl. H01] 61/20 high efficiency to be Obtained with a relatively low [58] Field of Search 313/184, 225, 227, minimum envelope temperature other discharge 313/228, 229 sustaining materials desirably are added to the foregoing essential constituents to modify the color of the [56] References Cited discharge UNITED STATES PATENTS 7 3,514,659 5/1970 Gunglc et al. 313/184 22 Claims, 3 Drawing Figures 2 14Q L i Q [L L] m 120 '1 3 O E Lu 2 F q J w 80 (I I I F 800 900 I000 MINIMUM ENVELOPE- TEMPERATURE C PATENTEUJAMSIHM 3.786297 MINIMUM ENvH UFE- TEMPERATURE "C CROSS-REFERENCETO RELATED APPLICATION This application is a continuation-in-part of copending application Ser. No. 108,427, filed Jan. 21, 1971, now abandoned titled Discharge Lamp Which Incorporates Cerium and Cesium l-Ialides and a High Mercury Loading," by the present applicants and owned by the present assignee.

BACKGROUND OF THE INVENTION This invention generally relates to discharge devices and, more particularly, to so-called metal halide discharge devices wherein predetermined amounts of selected discharge-sustaining materials are utilized in order to improve the performance of the device.

In US. Pat. No. 3,234,421 dated Feb. 8, 1966, is disclosed a so-called metal halide discharge lamp wherein selected metallic halides, particularly those of Group IA, IIA, HE, and IIIA are incorporated into the device in order to modify the color of the discharge and the operating efficiency of same with respect to the generation of visible light.

In U.S. Pat. No. 3,334,261 dated Aug. 1, 1967, is disclosed a metal-halide discharge device which incorporates rare-earth metal halides in order to modify the discharge. In U.S. Pat. No. 3,407,327 dated Oct. 22, 1968, is disclosed a metal-halide discharge device which incorporates a filling of alkali metal and scandium halide. In US. Pat. No. 3,514,659, dated May 26, 1970, is disclosed a metal-halide lamp which incorporates a relatively smallamount of cesium iodide in order to reduce the so-called reignition voltage, with the cesium halide being limited in amount so that it will not lower the arctemperature to a point where the emission lines. of the primary light emitting metals are weakened. In U.S. Pat. No. 3,262,012 dated July 19, 1.966 isdisclosed a discharge device which utilizes cesium and sodium halides in addition to other rare-earth metal halides. The device isintended to operate on a conventional ballast and the mercury loading which is utilized is quite low.

SUMMARY OF THE INVENTION An arc-discharge device comprises a sealed elongated light-transmitting envelope which encloses a predetermined volume. As is conventional, electrical leadin conductors are sealed through the envelope and are electrically connected to electrodes which are operatively spaced apart a predetermined distance within the envelope. A discharge-sustaining filling is enclosed by the envelope and contains the following as essential constituents: a small charge of inert ionizable starting gas; mercury in predetermined amount as required, when fully vaporized, to produce an operating mercury vapor pressure of from 3 to atmosheres; at least one of praseodymium halide, neodymium halide and cerium halide, excluding the fluoride, in total amount of from 2 X 10" to2.5 X 10 gram mol/cm of spacing between the lamp electrodes, and preferably from 1.4 X 10" to 5.4 X 10" gram mol/cm of spacing between the lamp electrodes; cesium halide excluding the fluoride, in amount of from 3.5 X 10" to 2.5 X 10" gram mol/cm of spacing between the lamp electrodes and 1 preferably from 3.5 X 10' to 5.4 X 10" gram mol/cm of spacing between the lamp electrodes; and the molar ratio of total praseodymium halide, neodymium halide and cerium halide to cesium halide is from 4/1 to 1/2.5. Other additives desirably are included with the discharge sustaining filling, such as sodium halide, dysprosium halide and samarium halide, with these additional added halides present in predetermined amount.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the invention, reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying drawings, in which: FIG. 1 is a discharge lamp, shown partly in section, which incorporates a quartz inner envelope and a discharge-sustaining filling in accordance with the present invention;

FIG. 2 is a modified form of discharge device showing only a sectional view of the inner arc tube which is formed of polycrystalline alumina or similar refractory envelope material and which incorporates a dischargesustaining filling in accordance with the present invention; and

FIG. 3 is a graph of relative efficiency versus minimum envelope temperature illustrating the lower envelope temperatures which are required to achieve optimum efficiency when utilizing a discharge-sustaining filling in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With specific reference to the form of the invention illustrated in the drawing, the discharge device or lamp 10 is generally similar in construction to the usual high pressure, mercury-vapor lamp and! comprises a radiation-transmitting sealed inner envelope or arc tube 12 having electrodes 14 operatively disposed proximate either end thereof and operable to sustain a vapor discharge therebetween. A charge of mercury 16 and a small, charge of inert ionizable. starting gas such as 20 torrs of argon are contained within the inner envelope 12. The charge of mercury 16 is present in predetermined amount as required, whenfully vaporized as the sole discharge-sustaining constituent, to provide an operating mercury-vapor pressure of from 3 to 15 atmospheres as calculated on the basis of an average temperature for the vaporized mercury of 2,000 K. This average temperature may vary somewhat depending upon the various discharge-sustaining constituents which are used and the lamp operating conditions, but this indicated figure is a representative average temperature for the vaporized mercury. Since the envelope volume is'always known, the required amount of mercury to provide the proper operating conditions can readilybe calculated. During operation of the device, the other discharge-sustaining materials may interact with the mercury to affect the actual operating pressure of the mercury and, in addition, the extreme temperature gradients from the actual arc to the envelope wall may have an effect on the actual pressure within the operating device. For this reason, it is more accurate to express the required amount of mercury as if that material, per se, werev the sole discharge-sustaining constituent, since the amounts of mercury placed into the arc tube are known and the resulting pressure, as deter mined by the foregoing mercury vapor temperature, can readily be ascertained.

At least one of praseodymium halide, neodymium halide and cerium halide l8, excluding the fluoride, is included within the arc tube 12 in total amount of from 2 X 10 to 2.5 X 10 and preferably from 1.4 X 10 to 5.4 X 10" gram mol/cm of spacing between the electrodes l4, and as a specific example, the electrodes are spaced from each other by a distance of 7 centimeters and the arc tube 12 encloses a volume of 20 cubic centimeters. Also included within the arc tube is cesium halide 20, excluding the fluoride, in amount of from 3.5 X 10 to 2.5 X 10* and preferably from 3.5 X 10 to 5.4 X l gram mol/cm of spacing between the electrodes 14. In accordance with the present invention, the molar ratio of total praseodymium halide, neodymium halide and cerium halide to cesium halide is from 4/1 to l/2.5.

A radiation-transmitting, sealed outer envelope 24 is spaced from and surrounds the arc tube and preferably the space between the arc tube 12 and the outer envelope 24 is evacuated. Electrical lead-in conductors 26 are sealed through both the inner arc tube 12 and the outer envelope 24 and serve to electrically connect the operating electrodes 14 to a conventional power source (not shown).

A starting electrode 30 is also included within the arc tube 12 and connects through a starting resistor 32 to one end of the electrical lead-in conductors 26. The arc tube 12 is maintained in spaced relationship from the outer envelope 24 by means of a conventional supporting frame 34. Ribbon conductors 36 serve to facilitate hermetically sealing the lead-in conductors through the ends of the arc tube. The lead-in conductors are sealed through the outer envelope 24 by means of a conventional re-entrant stem press 38 and connect to a standard mogul base 40 to facilitate electrical connection to the power'source. As a specific example, the lamp as shown is designed to operate with the power input of 500 watts.

In the alternative embodiment 42, as shown in FIG. 2, the arc tube or envelope 44 is a high density sintered polycrystalline alumina body which has alumina end caps 46 sealed thereto. The electrodes 48 are operatively positioned proximate the envelope ends. At the ends of the arc tube 44 there are provided exhaust and fill tubulations 50 which also serve the function of supporting the electrodes 48. In accordance with the present invention, the mercury 16, at least one of praseodymium iodide, neodymium iodide and cerium iodide 18 and also cesium iodide 20 are included within the art tube 44 in predetermined amount as specified for the previous embodiment, along with the small charge of inert ionizable starting gas. To complete this embodiment as an operative device, the arc tube 44 would normally be included within an outer envelope as in the embodiment shown in FIG. 1, and the general construction of such a device is well known.

, As outlined hereinbefore, the essential constituents which comprise the discharge-sustaining filling are the small charge of inert ionizable starting gas, mercury as required to provide an operating vapor pressure of mercury per se of from 3 to atmospheres, at least one of praseodymium halide, neodymium halide and cerium halide, excluding the fluoride, in predetermined amount and cesium halide, excluding the fluoride, in predetermined amount, with a specified relative molar ratio of these rare-earth metal halides to cesium halide. The functioning of the mercury is to provide adequate voltage drop or loading between the electrodes and in conjunction with the cesium halide, the relatively high mercury vapor pressure and cesium halide function to lower the minimum envelope temperature or so-called cold-spot temperature at which optimum lamp operating efficiency is obtained. The specified rare-earth metal halides function to produce an extremely efficient discharge, and cerium halide is preferred. As a specific example, for an arc tube which encloses a volume of approximately 20 cubic centimeters and an electrode spacing of 7 centimeters, mercury included in amount of approximately 200 mg. will provide a mercury operating pressure of approximately 8 atmospheres. Along with the mercury filling are included cerium triiodide in amount of 20 milligrams and cesium iodide (Csl) in amount of 10 milligrams. Alternatively, praseodymium triiodide in amount of 20 milligrams or neodymium triiodide in amount of 20 milligrams can be used to replace the cerium iodide in the foregoing example. These rare-earth metal iodides can be mixed, if desired.

In order to improve the color appearance and color rendering properties which are obtained from a discharge-sustaining filling as described hereinbefore, it is highly desirable to include one or more of the following halides, excluding the fluorides: sodium halide, dysprosium halide and/or samarium halide. The sodium halide provides radiations in the yellow-orange region of the visible spectrum, dysprosium halide provides a red and blue emission and samarium halide provides a bluishwhite emission. The supplemental rare-earth halides may be regarded as a partial substitute for the praseodymium, neodymium and/or cerium halides, although these supplemental halides desirably are present in total amount of at least 7 X 10" and preferably at least 1.4 X 10 gram mol/cm of electrode spacing. The sodium halide may be regarded as a partial substitute for the cesium halide, although the cesium halide should always be present in amount of at least 3.5 X 10* gram mol/cm of electrode spacing. The sodium halide desirably is present in amount of at least 2.5 X 10" and preferably at least 3.5 X 10" gram mol/cm of electrode spacing. Preferably, the supplemental rare-earth halides, as outlined hereinbefore, namely dysprosium halide and samarium halide, are present in gram mol amount which does not differ appreciably from the gram mol amounts of the praseodymium, neodymium and/or'cerium halides and the mol ratio of supplemental rare-earth halide to these required halides should not exceed 5/1. Preferably the sodium halide is present in gram mol amount which does not differ appreciably from the gram mol amount of the cesium halide and the mol ratio of sodium halide to cesium halide should not be greater than 5/1.

In FIG. 3, is shown a graph of relative efficiency versus minimum envelope temperature for various embodiments of discharge devices fabricated in accordance with the present invention, and equivalent performances for control devices are also shown. The curve designated A illustrates luminous efficiency versus minimum envelope temperature for an arc tube dosed with 20 milligrams cerium triiodide, 20 milligrams of dysprosium triiodide, 0.4 milligram of thallium metal, and 50 milligrams of mercury which as the sole discharge-sustaining material would provide an opi a straight line relationship.

erating mercury vapor pressure of approximately 2 atmospheres. The are tube had an electrode spacing of 7 centimeters and an arc tube volume of 20 cc. As shown, the graph of efficiency versus temperature is essentially a straight line relationship. Since, as a practical matter, the maximum practical envelope temperatures are limited, the efficiency which is obtainable from such a device is likewise limited.

In the curve designated B, 15 milligrams of cesium iodide was used to replace the thallium and, as shown, with an operating mercury pressure of approximately 2 atmospheres, a peak of efficiency is realized at a somewhat reduced minimum envelope temperature, with the optimum efficiency being realized at a minimum envelope temperature of approximately 910 Centigrade. Such a temperature is still unduly high, however, for use as a practical lamp embodiment.

In the curve designated C," the lamp envelope which had an electrode spacing of 7 cm. and a volume of 8 cc. was dosed with 10 milligrams of cerium triiodide and 10 milligrams of dysprosium triiodide along with 150 milligrams of mercury. This would provide an operating pressure for the mercury per se of approximately atmospheres. As shown, the graph of efficiency versus minimum envelope temperature again is In the curve designated D, the arc tube, which had an electrode spacing of 7 cm. and a volume of 8 cc., was dosed with 10 milligrams cerium triiodide, l0 milligrams dysprosium triiodide, 10 milligrams of cesium iodide, and 100 milligrams of mercury which if used as the sole discharge-sustaining material would provide an operating mercury vapor pressure of approximately 10 atmospheres. Optimum efficiency of approximately 155 lumens per watt was achieved with a minimum envelope temperature of approximately 665 C, which temperature can readily be achieved with a quartz arc tube. As the mercury pressure isincreased over that mercury pressure present in the lamp used in taking curve D, the minimum envelope temperature required for optimum efficiency is further decreased and the curve designated E represents the performance of a similar arc tube dosed with approximately 10 milligrams cesium iodide, 10 milligrams dysprosium iodide, 10 milligrams cerium triiodide, and 150 milligrams mercury which when utilized as the sole dischargesustaining constituent would provide an operating mercury vapor pressure of approximately 15 atmospheres. Optimum efficiency with this embodiment was achieved with a minimum envelope temperature of approximately 520" C, which temperature is readily obtainable with a standard lamp construction. While cerium iodide is preferred in combination with the cesium iodide and high mercury loading in order to otbain optimum efficiency with a relatively low minimum envelope temperature, as shown in the curves D and E of FIG. 3, the cerium iodide can be replaced by 'an equivalent gram mol amount of praseodymium iodide or neodymium'iodide. This will provide a high optimum efficiency with a lowering of the minimum envelope temperature.

As stated hereinbefore, in order to achieve a usable minimum envelope temperature, the pressure of the mercury vapor should be present in such an amount that if used alone as the discharge-sustaining filling, the mercury vapor pressure in an operating device would be from 3 to 15 atmospheres. For an arc tube having a volume of approximately 20 cc: as described in the preferred embodiment, the mercury dosing or loading should befrom milligrams to 375 milligrams. Preferably, the mercury vapor pressure during operation of the device should be from 4 atmospheres to 10 atmospheres which will require a mercury dose of from mgs. to 250 mgs. (5 mgs/cc to 12.5 mgs/cc) for the preferred embodiment as described hereinbefore.

In summary, the combination of the cesium halide plus the relatively high mercury loading, along with the praseodymium, neodymium and/or cerium iodide provides an efficient discharge and permits optimum efficiency to be realized with a relatively low envelope temperature. The mechanism by which this is achieved is not completely understood, but it appears that the increased mercury pressure in combination with the cesium halide increases the amount of the desired radiating species in the arc, and provides a more diffuse discharge mode which has a lower, more nearly optimum arc temperature. The resulting broader arc has less tendency to bow during operation and even when it does, this broader arc is less destructive to the envelope because it has a lower temperature and is less concentrated.

Other additional supplemental discharge-sustaining fillings can be used, if desired, further to modify the discharge. Examples of such additional halides, excluding the fluoride of course, are holmium halide, scandium halide, gadolinium halide and indium halide. The holmium provides red and blue supplemental radiations, the scandium provides strong blue and supplemental other radiations, the gadolinium provides a blue sup plemental radiation and the indium provides blue supplemental radiation. If these additional additives are used, the molar ratio of the holmiurn, scandium and the gadolinium halides with respect to the praseodymium, neodymium and/or cerium halide should be less than about 5/1 and the molar ratio of the indium halide to the rare-earth metal halide should be less than about I I. As a matter of practicability, any of these additives should be present in amount of at least about 2 X 10 and preferably about 1.4 X 10' gram mol/cm of spac ing between the arc tube electrodes, although lesser amounts can be used if desired.

While the foregoing examples have considered the iodides, it should be understood that the bromides or the chlorides in equivalent gram mol amount can be substituted therefor. Alternatively, any mixture of iodide, bromide and/or chloride can be utilized if desired. While the specific performance curves as shown in FIG. 3 are directed to a mixture of cesium iodide and cerium iodide with supplemental dysprosium iodide, any of the other supplemental halide additives will provide similar results, although the color of. the discharge and color rendering properties thereof will vary depending upon the supplemental additive and the amount which is utilized. Preferably, the supplemental additives are generally used in gram mol amounts not appreciably different from the gram mol amounts of praseodymium, neodymium and/or cerium halides, ex cept for the sodium halide supplemental additive which desirably is used in gram mol amount not appreciably different from the gram mol amount of cesium halide.

A particularly attractive combination of dischargesustaining fillings is a combination of cerium halide, sodium halide, samarium as the dihalide, and cesium halide. As a specific example, for an. arc tube having an electrode spacing of 7 cm. and enclosing a volume of 20 cc., the arc tube is dosed with 200 mg. mercury, 8 mg. cerium triiodide, mg. sodium iodide, 15 mg. samarium diiodide and 5 mg. cesium iodide. Argon starting gas is used at a pressure of 20 torrs. Such an embodiment operates with an efficiency of 140 lumens per watt when operated with a minimum envelope temperature of 560 C. The foregoing so--called dosing will provide a mercury vapor pressure of about 8 atmospheres, the cerium iodide and cesium iodide are present in amount of about 2 to 3 X gram mol/cm of electrode spacing, and the samarium iodide and sodium iodide are present in amount of about 5 X 10 gram mol/cm of electrode spacing. The color of the discharge approximates a white color and the color rendition of illuminated objects is good.

The total amount of iodide dosing in the foregoing specific lamp combination is 33 mgs. It has been found that this total iodide dosing can be substantially reduced without materially affecting the performance of the lamp. As anexample, for the foregoing specific lamp, the total amount of iodide dosing can be reduced to 10 mgs. or even less, while still maintaining the total mercury dosing at about 200 mgs. For. best performance, however, the relative gram mol amounts of the iodide additives should be maintained at about the same relative molar ratio. More specifically, the relative amounts of the indicated iodides are preferably maintained in about the following relative gram mol proportions: cerium iodide, 2 to 3; cesium iodide, 2 to 3; samarium iodide, 4 to 6; and sodium iodide, 4 to 6.

We claim as our invention:

1. An arc-discharge device comprising a sealed elongated light-transmitting envelope which encloses a predetermined volume, electrical lead-in conductors sealed through said envelope and electrically connected to electrodes which are operatively spaced apart a predetermined distance within said envelope, a discharge-sustaining filling enclosed by said envelope and having the following as essential constituents: a small charge of inert ionizable starting gas; mercury in predetermined amount as required, when fully vaporized as the sole discharge-sustaining constituent, to provide an operating mercury-vapor pressure of from 3 to atmospheres as calculated on the basis of an average mercury vapor temperature of 2,000" K; at least one of praseodymium halide, neodymium halide and cerium halide, excluding the fluoride, in total amount of from 1.4 X 10 to 5.4 X 10" gm mol/cm of spacing between said electrodes; cesium halide, excluding the fluoride, in amount of from 3.5 X 10 to 5.4 X 10 gm mol/cm of spacing between said electrodes; and the molar ratio of said praseodymium halide, neodymium halide and cerium halide to said cesium halide is from 4/1 to l/2.5.

2. The arc-discharge device as specified in claim 1, wherein said mercury is present in predetermined amount as required when fully vaporized to produce an operating mercury vapor pressure of from about 4 to about 10 atmospheres.

3. The arc-discharge device as specified in claim 2, wherein said halides are the iodides.

4. The arc-discharge device as specified in claim 2, wherein said mercury is present in amount of from 5 to 12.5 mgs/cc of said predetermined volume of said envelope.

5. The arc-discharge device as specified in claim 1, wherein there is also included within said envelope as supplemental discharge-sustaining filling at least one halide, excluding the fluoride, of sodium halide, dysprosium halide, and samarium halide, the molar ratio of said dysprosium halide and said samarium halide to said praseodymium and neodymium and cerium halide being less than about 5/l, and the molar ratio of said sodium halide to said cesium halide being less than about 5/1.

6. The arc-discharge device as specified in claim 5, wherein said dysprosium halide and said samarium halide are present in amount of at least 1.4 X 10 gm mol/cm of spacing between said electrodes, and said sodium halide is present in amount of at least 3.5 X 10 gm mol/cm of spacing between said electrodes.

7. The arc-discharge device as specified in claim 5, wherein said halides are the iodides.

8. The arc-discharge device as specified in claim 5, wherein said supplemental discharge-sustaining filling is samarium as the dihalide and sodium halide.

9. The arc-discharge device as specified in claim 8, wherein said mercury vapor pressure is about 8 atmospheres, cerium is present as the triiodide in amount of about 2 to 3 X 10" gram mol/cm of spacing between said electrodes, cesium is present as the iodide in amount of 2 to 3 X 10 gram mol/cm of spacing between said electrodes, samarium is present as the diiodide in amount of about 5 X 10" gram mol/cm of spacing between said electrodes, and sodium is present as the iodide in amount of about 5 X 10 gram mol/cm of spacing between said electrodes.

10. The arc-discharge device as specified in claim 6, wherein there is also included within said envelope an additional supplemental discharge-sustaining filling at least one halide, excluding the fluoride, of holmium halide, scandium halide, gadolinium halide and indium halide, the molar ratio of said holmium and scandium and gadolinium halides to said praseodymium and neodymium and cerium halides is less than about 5/1, and the molar ratio of said indium halide to said praseodymium and neodymium and cerium halides is less than about 1/1.

11. The arc-discharge device as specified in claim 10 wherein said scandium halide, said holmium halide, said gadolinium halide and said indium iodide are present in amount of at least 1.4 X 10 gm mol/cm of spacing between said electrodes.

12. The arc-discharge device as specified in claim 1 1, wherein said halides are the iodides.

13. An arc-discharge device comprising a sealed elongated light-transmitting envelope which encloses a predetermined volume, electrical lead-in conductors sealed through said envelope and electrically connected to electrodes which are operatively spaced apart a predetermined distance within said envelope, a discharge-sustaining filling enclosed by said envelope and having the following as essential constituents: a small charge of inert ionizable starting gas; mercury in predetennined amount as required, when fully vaporized as the sole discharge-sustaining constituent, to provide an operating mercury-vapor pressure of from 3 to 15 atmospheres as calculated on the basis of an average mercury vapor temperature of 2,000 K, at least one of praseodymium halide, neodymium halide and cerium halide, excluding the fluoride, in total amount of from 2 X 10" to 2.5 X 10 gm mol/cm of spacing between said electrodes; cesium halide, excluding the fluoride, in amount of from 3.5 X 10 to 2.5 X 10 gm mol/cm of spacing between said electrodes; and the molar ratio of said praseodymium halide, neodymium halide and cerium halide to said cesium halide is from 4/1 to 1/2.5.

14. The arc-discharge device as specified in claim 13, wherein said mercury is present in predetermined amount as required when fully vaporized to produce an operating mercury vapor pressure of from about 4 to about 10 atmospheres.

15. The arc-discharge device as specified in claim 14, wherein said halides are the iodides.

16. The arc-discharge device as specified in claim 14, wherein said mercury is present in amount of from 5 to 12.5 mgs/cc of said predetermined volume of said envelope.

17. The arc-discharge device as specified in claim 13, wherein there is also included within said envelope as supplemental discharge-sustaining filling at least one halide, excluding the fluoride, of sodium halide, dysprosium halide, and samarium halide, the molar ratio of said dysprosium halide and said samarium halide to said praseodymium and neodymium and cerium halide being less than about 5/1, and the molar ratio of said sodium halide to said cesium halide being less than about 5/1.

18. The arc-discharge device as specified in claim 17, wherein said dysprosium halide and said samarium halide are present in amount of at least 7 X 10 gm mol/cm of spacing between said electrodes, and said sodium halide is present in amount of at least 2.5 X 10 gm mol/cm of spacing between said electrodes.

19. The arc-discharge device as specified in claim l7,

wherein said halides are the iodides.

20. The arc-discharge device as specified in claim 17, wherein said supplemental discharge-sustaining filling is samarium as the dihalide and sodium halide.

21. The arcdischarge device as specified in claim 20, wherein said mercury vapor pressure is about 8 atmospheres, and cerium as the triiodide, cesium as the iodide, samarium as the diiodide and sodium as the iodide are present in about the following proportions, as expressed in terms of relative gram mo] amounts: cerium iodide, 2 to 3; cesium iodide, 2 to 3; samarium iodide, 4 to 6; and sodium iodide, 4 to 6.

22. An arc-discharge device comprising a sealed elongated light-transmitting envelope which encloses a predetermined volume, electrical lead-in conductors sealed through said envelope and electrically connected to electrodes which are operatively spaced apart a predetermined distance within said envelope, a discharge-sustaining filling enclosed by said envelope and having the following as essential constituents: a small charge of inert ionizable starting gas; mercury in predetermined amount as required, when fully vaporized as the sole discharge-sustaining constituent, to provide an operating mercury-vapor pressure of from 3 to 15 atmospheres as calculated on the basis of an average mercury vapor temperature of 2,000 K; cerium halide, excluding the fluoride, in total amount of from 1.4 X 10 to 5.4 X 10 gm mol/cm of spacing between said electrodes; cesium halide, excluding the fluoride, in amount of from 3.5 X 10 to 5.4 X 10 gm mol/cm of spacing between said electrodes; and the molar ratio of said cerium halide to said cesium halide is from 4/1 to 1/2.5. 

2. The arc-discharge device as specified in claim 1, wherein said mercury is present in predetermined amount as required when fully vaporized to produce an operating mercury vapor pressure of from about 4 to about 10 atmospheres.
 3. The arc-discharge device as specified in claim 2, wherein said halides are the iodides.
 4. The arc-discharge device as specified in claim 2, wherein said mercury is present in amount of from 5 to 12.5 mgs/cc of said predetermined volume of said envelope.
 5. The arc-discharge device as specified in claim 1, wherein there is also included within said envelope as supplemental discharge-sustaining filling at least one halide, excluding the fluoride, of sodium halide, dysprosium halide, and samarium halide, the molar ratio of said dysprosium halide and said samarium halide to said praseodymium and neodymium and cerium halide being less than about 5/1, and the molar ratio of said sodium halide to said cesium halide being less than about 5/1.
 6. The arc-discharge device as specified in claim 5, wherein said dysprosium halide and said samarium halide are present in amount of at least 1.4 X 10 6 gm mol/cm of spacing between said electrodes, and said sodium halide is present in amount of at least 3.5 X 10 7 gm mol/cm of spacing between said electrodes.
 7. The arc-discharge device as specified in claim 5, wherein said halides are the iodides.
 8. The arc-discharge device as specified in claim 5, wherein said supplemental discharge-sustaining filling is samarium as the dihalide and sodium halide.
 9. The arc-discharge device as specified in claim 8, wherein said mercury vapor pressure is about 8 atmospheres, cerium is present as the triiodide in amount of about 2 to 3 X 10 6 gram mol/cm of spacing between said electrodes, cesium is present as the iodide in amount of 2 to 3 X 10 6 gram mol/cm of spacing between said electrodes, samarium is present as the diiodide in amount of about 5 X 10 6 gram mol/cm of spacing between said electrodes, and sodium is present as the iodide in amount of about 5 X 10 6 gram mol/cm of spacing between said electrodes.
 10. The arc-dischargE device as specified in claim 6, wherein there is also included within said envelope an additional supplemental discharge-sustaining filling at least one halide, excluding the fluoride, of holmium halide, scandium halide, gadolinium halide and indium halide, the molar ratio of said holmium and scandium and gadolinium halides to said praseodymium and neodymium and cerium halides is less than about 5/1, and the molar ratio of said indium halide to said praseodymium and neodymium and cerium halides is less than about 1/1.
 11. The arc-discharge device as specified in claim 10 wherein said scandium halide, said holmium halide, said gadolinium halide and said indium iodide are present in amount of at least 1.4 X 10 6 gm mol/cm of spacing between said electrodes.
 12. The arc-discharge device as specified in claim 11, wherein said halides are the iodides.
 13. An arc-discharge device comprising a sealed elongated light-transmitting envelope which encloses a predetermined volume, electrical lead-in conductors sealed through said envelope and electrically connected to electrodes which are operatively spaced apart a predetermined distance within said envelope, a discharge-sustaining filling enclosed by said envelope and having the following as essential constituents: a small charge of inert ionizable starting gas; mercury in predetermined amount as required, when fully vaporized as the sole discharge-sustaining constituent, to provide an operating mercury-vapor pressure of from 3 to 15 atmospheres as calculated on the basis of an average mercury vapor temperature of 2,000* K, at least one of praseodymium halide, neodymium halide and cerium halide, excluding the fluoride, in total amount of from 2 X 10 7 to 2.5 X 10 4 gm mol/cm of spacing between said electrodes; cesium halide, excluding the fluoride, in amount of from 3.5 X 10 7 to 2.5 X 10 4 gm mol/cm of spacing between said electrodes; and the molar ratio of said praseodymium halide, neodymium halide and cerium halide to said cesium halide is from 4/1 to 1/2.5.
 14. The arc-discharge device as specified in claim 13, wherein said mercury is present in predetermined amount as required when fully vaporized to produce an operating mercury vapor pressure of from about 4 to about 10 atmospheres.
 15. The arc-discharge device as specified in claim 14, wherein said halides are the iodides.
 16. The arc-discharge device as specified in claim 14, wherein said mercury is present in amount of from 5 to 12.5 mgs/cc of said predetermined volume of said envelope.
 17. The arc-discharge device as specified in claim 13, wherein there is also included within said envelope as supplemental discharge-sustaining filling at least one halide, excluding the fluoride, of sodium halide, dysprosium halide, and samarium halide, the molar ratio of said dysprosium halide and said samarium halide to said praseodymium and neodymium and cerium halide being less than about 5/1, and the molar ratio of said sodium halide to said cesium halide being less than about 5/1.
 18. The arc-discharge device as specified in claim 17, wherein said dysprosium halide and said samarium halide are present in amount of at least 7 X 10 7 gm mol/cm of spacing between said electrodes, and said sodium halide is present in amount of at least 2.5 X 10 7 gm mol/cm of spacing between said electrodes.
 19. The arc-discharge device as specified in claim 17, wherein said halides are the iodides.
 20. The arc-discharge device as specified in claim 17, wherein said supplemental discharge-sustaining filling is samarium as the dihalide and sodium halide.
 21. The arc-discharge device as specified in claim 20, wherein said mercury vapor pressure is about 8 atmospheres, and cerium as the triiodide, cesium as the iodide, samarium as the diiodide and sodium as the iodide are Present in about the following proportions, as expressed in terms of relative gram mol amounts: cerium iodide, 2 to 3; cesium iodide, 2 to 3; samarium iodide, 4 to 6; and sodium iodide, 4 to
 6. 22. An arc-discharge device comprising a sealed elongated light-transmitting envelope which encloses a predetermined volume, electrical lead-in conductors sealed through said envelope and electrically connected to electrodes which are operatively spaced apart a predetermined distance within said envelope, a discharge-sustaining filling enclosed by said envelope and having the following as essential constituents: a small charge of inert ionizable starting gas; mercury in predetermined amount as required, when fully vaporized as the sole discharge-sustaining constituent, to provide an operating mercury-vapor pressure of from 3 to 15 atmospheres as calculated on the basis of an average mercury vapor temperature of 2,000* K; cerium halide, excluding the fluoride, in total amount of from 1.4 X 10 6 to 5.4 X 10 5 gm mol/cm of spacing between said electrodes; cesium halide, excluding the fluoride, in amount of from 3.5 X 10 7 to 5.4 X 10 5 gm mol/cm of spacing between said electrodes; and the molar ratio of said cerium halide to said cesium halide is from 4/1 to 1/2.5. 